Quantitative Assessment of Core Fucosylation for Congenital Disorders of Glycosylation

Congenital disorders of glycosylation (CDG) include a group of diseases characterized by defects of N-glycan fucosylation. The analytical molecule of choice for the diagnosis of CDG affecting N-glycosylation is serum transferrin: approximately 10% of the glycans attached to transferrin are fucosylated via an α1,6 linkage at the innermost N-acetylglucosamine residue, termed "core fucosylation." Isoelectric focusing (IEF) of transferrin is often used for diagnosis, but IEF is ineffective in detecting abnormal fucosylation. Here, we present mass spectrometry (MS) methods for detecting fucosylation disorders. First, the level of core fucosylation of the glycan attached to Asn630 of transferrin can be measured by the signal intensity ratio of tryptic peptide ions containing fucosylated and nonfucosylated biantennary oligosaccharides. The core fucosylation level at this glycosylation site in the 0- to 32-year-old group (n = 68) was 7.9 ± 1.7 (%, mean ± SD), and nearly null for SLC35C1-CDG caused by defects in the GDP-fucose transporter. More simply, fucosylation levels can be measured by quadrupole time-of-flight (QTOF) MS of intact transferrin. The fucosylation levels of intact transferrin measured by MS with a Q-mass analyzer, which is currently used as an instrumental standard for newborn screening for inborn errors of metabolism and has a lower resolution than the QTOF analyzer, correlated well with the values obtained by glycopeptide analysis. These methods, namely the analysis of glycopeptides or intact transferrin by Q MS, can also be used on dried blood spots and are expected to help facilitate the diagnosis of CDG affecting N-glycan fucosylation.